Neuroendocrine control of sound production and of sexually dimorphic nuclei, clusters of neurons in the CNS, of toadfish, frogs, birds and mammals are important avenues for understanding neural control of behavior. The stereotypy and simplicity of the male toadfish's mating call, which is determined by rate and duration of sonic muscle contraction, make the call an attractive model for neuroendocrine and physiological control of sexually-related vocal behavior. Fruitful exploitation of this model requires more detailed knowledge of sonic neuroanatomy, a major goal of this proposal. Demonstration in our laboratory of a multiyear four fold increase in number and size of neurons in the sonic motor nucleus (SMN), a likely homologue of the hypoglossal nucleus, has implications for neuroplasticity and CNS regeneration, since neuron number and size are fixed pre- or perinatally in mammals. Additionally, we have divided toadfish into 2 classes based on SMN neuron size: (1) females and males with small neurons and (2) males with large neurons. This intrasexual dimorphism suggests hypotheses which will allow correlation of neuron morphology with social behavior. Using primarily anatomical methods, this proposal has 3 major related foci: (1) morphological basis for ontogeny and sexual dimorphism of sonic muscles (hypertrophy and hyperplasia of muscle fibers); (2) morphology, histochemistry, ontogeny, and sexual dimorphism of the SMN in embryonic and adult toadfish and correlates with social behavior; and (3) identification of sonic pathways from the preoptic area to the SMN with HRP, and the comparison of sonic nuclei with nuclei containing 3H steroid concentrating neurons, which will be established by autoradiography.